The confiscated pills were marked as hydrocodone, a commonly prescribed drug for moderate pain relief.
Investigators looking into pop legend Prince's death believe that counterfeit pills may have killed him, a report said Monday.
The Star Tribune, the daily newspaper in Prince's hometown of Minneapolis, said that authorities turned to the theory after seizing pills from Prince's Paisley Park estate.
The confiscated pills were marked as hydrocodone, a commonly prescribed drug for moderate pain relief, but in fact contained fentanyl, an intensely powerful painkiller for which Prince had no prescription.
An autopsy earlier found that Prince died on April 21 from an accidental overdose of fentanyl but did not address how he obtained the drug.
The Star Tribune, which quoted an anonymous source, said that investigators were "leaning toward the theory that he took the pills not knowing they contained the drug."
Prince was found dead in his Paisley Park estate just as he prepared to discuss treatment with a California expert in painkiller addiction.
The 57-year-old "Purple Rain" star was outwardly a model of health who was known to be conscientious in his food choices and critical of musicians who abused drugs.
But Prince was private about his personal life and was believed to be quietly in pain from a hip operation after years of awe-inspiring stage moves.
One of the most influential artists of his generation, the sprightly rocker was just five-foot-two (1.6-meter) and 112 pounds (51 kilograms).
In his death, Prince became the highest-profile victim of a US epidemic of painkiller addiction.
The US Drug Enforcement Administration in a report last month said that fentanyl was more widely available and killing more people than at any point since the drug's creation in 1959.
The federal agency said that hundreds of thousands of counterfeit painkiller pills were on the US market, many containing fentanyl that was mass-produced in labs in China.